As I have read in the literature, the virial radius of a dark matter halo is the radius of a sphere such that the density $\frac{M_{\text{virial}}}{\frac{4\pi}{3}R_{\text{virial}}^{3}}=97.2\rho_{\text{crit}},$ where $\rho_{\text{crit}}$ is the critical density of the Universe.

Does it mean that the dark matter halo has a finite extension given by $R_{\text{vir}}$?

Does it mean that outside this virial radius there's not dark matter anymore?


It simply means the radius within which the "virial equilibrium" holds. Beyond this radius, dark matter is still present, but it has low enough density to blend with the background matter in the universe.

The factor of $97.2$ in that equation however is arbitrary. It can range from anywhere between $50-200$.


Virial Mass

Virial Theorem#Astrophysics

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  • $\begingroup$ thanks for the answer... but if it is true, does it mean that outside a galaxy one will never get vacuum solutions of GR as dark matter will be always present? $\endgroup$ – Ernesto Lopez Fune Jul 3 '17 at 9:43

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